The Earth has seasons because it holds the same tilt throughout its revolution around the sun. Since this may sound confusing, let´s take the ceiling fan in my bedroom. The light in the middle is the sun. Now if we place a small ball, such as a tennis ball (Our Earth), and place it on one of the arms of the fan, two things will happen. 1: It will probably fall off. 2: Assuming it doesn´t fall off (which we will have to do), it will be going around our sun. This is what the term revolution refers to. Now, imagine I stab a skewer through the tennis ball. This is the Earth´s axis. The Earth spins around this axis at the same time as it goes around the sun. This axis is tilted, all the time, at exactly 23.5 degrees. You might wonder how it´s tilted, as in what it keeps the same angle with. Surprisingly enough, it doesn´t stay with the sun. The object it keeps the same angle with is the walls of my bedroom, not the sun. Because of this, sometimes the Earth is tilted at different angles with the sun. We will draw a line parallel to the axis, called the equator. The equator splits the Earth in half this way. The side north of the equator is the northern hemisphere, and the opposite side is the southern hemisphere. If we go back to the Earth tilting, we realize that as the Earth goes around the sun, keeping the same angle, the northern hemisphere tilts back and forth, away and towards the sun. When you shine a light directly at something, it gets all the light directly as well. When you use a flashlight with a circular end and point at a dark piece of paper, the result is a circle of light. But when you point it at a piece of paper at an angle, you get an oval. Since this other has a bigger area than the source, the energy has to spread out, and so there is less energy in each selected point. When, instead of a paper, we go back to the tennis ball, we see that the same principle is relevant. When the Southern Hemisphere is tilted away from the sun, the heat energy is spread out, and it is colder, creating Winter. Likewise, at the same time, the northern hemisphere is tilted towards the sun, and so the energy is more concentrated, creating summer. At one point they have to be at an epiphany where there is a maximum of sunlight on one hemisphere and the opposite on the other. When your hemisphere has the longest day possible we call it the summer solstice, and when you have the shortest day of the year it is called the winter solstice. On the summer solstice, a small area has daylight all day from the tilt. This region is called the arctic circle. The opposite region that would get 0 daylight is called the antarctic circle. During these maximums and minimums, there has to be a line around the Earth that has the most direct contact from the sun, 23.5 degrees from the equator either way. The northern one is called the tropic of Capricorn and the southern is the tropic of Cancer. Along with the solstices, there are equinoxes. These are the opposite of a solstice. Everywhere on Earth gets the same amount of sunlight, and the two equinoxes are practically the same, instead of opposites (like how the solstices are related to each other). At this point, the angle is not away or towards the sun. If you take the same pice of paper, and instead of tilting the paper, you spin it in a circle (clockwise or counterclockwise) compared to yourself, nothing happens to the beam of light. So even though the Earth is still tilted, it isn´t affecting the way the sun´s rays hit it. At these points, since none of the hemispheres are hogging the energy, they don´t have summer, and since the other isn´t having its light as spread, there isn´t winter. And so, the Earth has seasons because it holds the same tilt throughout its revolution around the sun.